Upthrust is the resultant upward force that a fluid exerts on an object immersed in it.

In terms of pressure, why does a submerged object experience an upthrust?

Pressure in a fluid increases with depth, so the upward force on the lower surface of the object is greater than the downward force on its upper surface, giving a resultant force upwards.

An object of volume 2.0 × 10⁻³ m³ is completely submerged in water. What is the upthrust on it, taking g = 9.81 N kg⁻¹?

Upthrust = ρVg = 1.0 × 10³ × 2.0 × 10⁻³ × 9.81 = 19.6 N.

Under what condition does an object float in a fluid?

It floats when it can displace fluid whose weight equals its own weight, which happens when its average density is less than the density of the fluid.

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Materials

Upthrust and Archimedes' principle

3 things to know
Papers 2 and 3

Overview

Recall questions on what upthrust is, Archimedes' principle, why a pressure difference produces it, and writing it in terms of the fluid density and volume displaced.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What is upthrust?

Process

What does Archimedes' principle state?

Process

How is upthrust written in terms of the density of the fluid and the volume displaced?

Questions from this subtopic

Each one with the model answer underneath.

1
What is upthrust?
Model answer
Upthrust is the resultant upward force that a fluid exerts on an object immersed in it.
2
What does Archimedes' principle state?
Model answer
The upthrust on an object is equal to the weight of the fluid that the object displaces.
3
In terms of pressure, why does a submerged object experience an upthrust?
Model answer
Pressure in a fluid increases with depth, so the upward force on the lower surface of the object is greater than the downward force on its upper surface, giving a resultant force upwards.
4
How is upthrust written in terms of the density of the fluid and the volume displaced?
Model answer
Upthrust = ρVg, where ρ is the fluid's density and V is the volume of fluid displaced.
5
An object of volume 2.0 × 10⁻³ m³ is completely submerged in water. What is the upthrust on it, taking g = 9.81 N kg⁻¹?
Model answer
Upthrust = ρVg = 1.0 × 10³ × 2.0 × 10⁻³ × 9.81 = 19.6 N.
6
Under what condition does an object float in a fluid?
Model answer
It floats when it can displace fluid whose weight equals its own weight, which happens when its average density is less than the density of the fluid.

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